1. Field of the Invention
The invention relates generally to an idle-speed control unit and an ejector system for a vehicle. More specifically, the invention relates to an idle-speed control unit and an ejector system for a vehicle, which appropriately suppress fluctuations in the idle speed even if the ejector is caused to operate or caused to stop operating.
2. Description of the Related Art
Conventionally, an ejector is used to supply a brake booster with a negative pressure of which the absolute value is larger than the absolute value of a negative pressure to be introduced from an intake passage of an intake system of an internal combustion engine, which provides communication between the atmosphere and each cylinder (hereinafter, simply referred to as an “intake system of an internal combustion engine” where appropriate). The ejector is usually arranged in a bypass passage that allows the intake air to bypass a throttle valve, and generates a negative pressure having a large absolute value with Venturi effect. Such ejector is described in the following publications. Japanese Patent Application Publication No. JP-2005-69175 (JP-A-2005-69175) describes a control apparatus for a vehicle, which includes a correction device that corrects the flow-rate at which the air to be taken in an internal combustion engine flows (hereinafter, sometimes referred to as the “intake air flow-rate”) based on the operating state of an ejector. Also, there is proposed a technology in which an ejector is arranged together with an idle-speed control valve in an idle duct, which allows the intake air to bypass a throttle valve, to form a negative pressure generator.
Japanese Patent Application Publication No. 2004-299567 (JP-A-2004-299567) describes a negative pressure generator that has the structure in which an ejector and an idle-speed control valve are combined with each other. Japanese Patent Application Publication No. 2005-201196 (JP-A-2005-201196) describes a negative pressure supply device for a vehicle formed by arranging a throttle valve for an ejector, which is fitted to a support shaft that rotates together with the throttle valve, in a bypass passage in which an ejector is provided.
When an internal combustion engine is idling, the idle-speed control is usually executed. In the idle-speed control, a flow-rate adjustment device such as an idle-speed control valve or a throttle valve is controlled to control the idle speed. FIG. 15 is the view conceptually showing the common idle-speed control. The idle-speed control usually includes the feedback control for controlling the flow-rate adjustment device so that fluctuations in the idle speed of the internal combustion engine are suppressed; the learning control for controlling the flow-rate adjustment device based on the results of the feedback control so that the idle speed is maintained at the target speed; and the correction control for controlling the flow-rate adjustment device so that the target speed is changed based on the operating state of, for example, an air-conditioner. Under the idle-speed control, the intake air flow-rate is adjusted to the required intake air flow-rate, which is required to operate the internal combustion engine at the target speed, by executing the controls described above. Accordingly, as shown in FIG. 15, when the ejector is caused to operate while the internal combustion engine is idling, the intake air flow-rate increases. At the same time, the intake air flow-rate is decreased by the feedback control to suppress fluctuations in the idle speed. In the feedback control executed at this time, the control amount to be achieved by the feedback control (hereinafter, sometimes referred to as the “feedback control amount”) is decreased by the correction amount corresponding to an increase in the intake air flow-rate (hereinafter, sometimes referred to as the “feedback correction amount”).
FIG. 13 is the graph schematically showing a change that occurs in the flow-rate of the intake air flowing through a bypass passage when an ejector is caused to operate. The cross-section of a passage formed within the ejector is gradually decreased toward the portion at which a negative pressure is generated with venturi effect. Accordingly, when the ejector is caused to operate, the intake air flow-rate increases not instantaneously but gradually. As a result, a time lag is caused between when the ejector is caused to operate and when the intake air flow-rate reaches the final value. However, Japanese Patent Application Publication No. 2005-69175 (JP-A-2005-69175) does not describe the manner in which the intake air flow-rate increases. Accordingly, it is considered that the intake air flow-rate is decreased uniformly through correction when the ejector is caused to operate, in the control apparatus for a vehicle described in JP-A-2005-69175. Namely, with the control apparatus for a vehicle described in JP-A-2005-69175, although fluctuations in the intake air flow-rate are ultimately suppressed, the intake air flow-rate may be temporarily decreased if the correction is made at an inappropriate time when the intake air flow-rate is transiently changing.
When the intake air flow-rate is transiently changing, controlling the air-fuel ratio accurately is likely to be difficult due to the delayed response to the detection of the intake air flow-rate. In contrast, with the control apparatus for a vehicle according to JP-A-2005-69175, even if the ejector is caused to operate or caused to stop operating when the engine is idling, for example, the detected intake air flow-rate is corrected so as to coincide with the intake air flow-rate that is actually increasing or decreasing. Accordingly, the inconvenience caused by the delayed response to the detection of the intake air flow-rate is minimized. As a result, the air-fuel ratio is controlled more accurately.
Meanwhile, the feedback control in the idle-speed control described above is usually executed based on the difference between the required intake air flow-rate and the detected intake air flow-rate. For example, if the detected intake air flow-rate is corrected by the control apparatus for a vehicle described in JP-A-2005-69175, the idle-speed control is more appropriately executed even if the ejector is caused to operate or caused to stop operating, because the inconvenience caused by the delayed response to the detection of the intake air flow-rate is minimized. However, the feedback control is executed in the idle-speed control. Accordingly, if the intake air flow-rate is corrected in a certain manner, the idle speed may fluctuate due to the feedback control if the ejector is caused to operate or caused to stop operating. In this case, such fluctuations may give a sense of discomfort to the driver.
As shown in FIG. 15, in the learning control, the control amount to be achieved by the learning control (hereinafter, sometimes referred to as the “learning control amount”) is decreased or increased by an amount corresponding to an increase or a decrease in the feedback control amount (hereinafter, sometimes referred to as “learning is executed”). At the same time, the feedback control amount is increased or decreased by an amount corresponding to a decrease or an increase in the learning control amount. However, when the intake air flow-rate is transiently changing, the learning is not always properly executed as intended. Therefore, if the learning is executed even when the ejector is caused to operate, the learning control amount may be considerably small. In this case, when the ejector is caused to stop operating, the intake air flow-rate considerably decreases, and the idle speed also considerably decreases. In addition, the intake air flow-rate becomes severely deficient. In some cases, the feedback control fails to be executed in time, and therefore engine stalling may occur.
In a negative pressure generator described in each of Japanese Patent Application Publication No. 2004-285838 (JP-A-2004-285838) and Japanese Patent Application Publication No. 2004-299567 (JP-A-2004-299567), an ejector is structured to generate a negative pressure in accordance with the intake air flow-rate adjusted by an idle-speed control valve. Accordingly, if a negative pressure having a large absolute value needs to be generated by the ejector, the idle speed inevitably excessively increases due to the structure. In this case, because a negative pressure to be introduced from an intake system of an internal combustion engine is decreased, a negative pressure generated by the ejector is decreased by an amount corresponding to a decrease in the negative pressure to be introduced from the intake system. Namely, due to the structure of the negative pressure generator described above, the ejector is not efficiently used when the absolute value of the negative pressure to be introduced from the intake system of the internal combustion engine is large. In the negative pressure supply device described in JP-A-2005-201196, the throttle valve and the throttle valve for an ejector cannot be controlled independently of each other. Accordingly, it is considered that the ejector is not efficiently used when the absolute value of the negative pressure to be introduced from the internal combustion engine is large. Meanwhile, the amount of negative pressure supplied by the ejector per unit time is not considerably large. Accordingly, a required negative pressure may not be obtained in time.